When a conventional wheeled vehicle is driven, an internal combustion engine of the vehicle drive system develops a torque which is applied to an engine shaft, transferred via a clutch to a gear shaft, and then delivered to an axle drive shaft via a transmission. On the other hand, when the vehicle enters an idling state, torque transfer in the drive train is interrupted, for example, by engaging the clutch or shifting to neutral. During idling, the internal combustion engine is not used to drive the vehicle and, thus, the internal combustion engine experiences a reduced load. However, even in the idling state, the internal combustion engine must still carry a load resulting from both its own internal friction and the mechanical and electrical auxiliary units connected to the engine shaft (e.g., pumps, servo drives, and/or a generator).
In order to keep fuel consumption, pollutant emissions and noise development low, the idle speed should be set as low as possible. However, the idle speed can only be reduced to a certain speed (i.e., the so-called critical speed). Below this critical speed, the internal combustion engine haltingly turns or even stops, since the instantaneously stored kinetic rotational energy is not sufficient to apply the compression and acceleration work essential for a compression stroke. Idling, in the broader sense, is therefore understood to mean any operating condition of the drive system in which the internal combustion engine has no vehicle drive function and runs at the lowest possible speed close to and above the critical speed.
In ordinary drive systems, the idling or idle speed is controlled by changing the supplied amount of air and/or the air-fuel ratio and/or the ignition point. By controlling these factors, the idle speed should then assume a specific set value (the so-called set idle speed) regardless of the instantaneously present operating conditions and regardless of load fluctuations associated with the auxiliary units. The set idle speed is located at a certain safety margin above the critical speed. The safety margin is essential even if no load fluctuations are expected during idling since ordinary idle controls do not guarantee very accurate constancy of idle speed at the set value. Since idling speed variations are present even in the absence of a load fluctuation, a safety margin must be present to rule out the possibility of falling short of the critical speed,. The effect of load fluctuations on the idle speed is then added to the safety margin.
It should be noted that the term "control" as used throughout the present text is understood in a broad sense, which embraces the terms "open loop control" (i.e., influencing a quantity in an open control loop or action chain) and "regulation" (i.e., influencing a quantity based on comparison with another stipulated quantity (i.e., closed loop control)). The same broad definition applies for derived terms, like "to control", "control device", etc.
Although the ordinary approaches to idle speed control mentioned above do function in principle, they suffer from certain drawbacks. For example, controlling idle speed by adjusting the air supply or air-fuel ratio (so-called "filling intervention") is relatively slow such that, after a deviation, a relatively long time passes before the set idle speed is reached again. This is disadvantageous because, in order to be able to rapidly counteract a change in the idle speed (and especially to avoid falling short of the critical speed), the torque of the internal combustion engine must be raised very quickly.
Special control methods which lead to more rapid adjustment of the idle speed have therefore already been proposed in the prior art. These prior art control methods are based, for example, on so-called disturbance variable compensation (a disturbance, for example, is a large consumer of power). Such filling intervention control also requires a relatively large safety margin from the critical speed and, thus, a relatively high idle speed.
Control based on adjustment of the ignition point (so-called "ignition intervention") permits very rapid return of the idle speed to the set idle speed. Ignition intervention is based on the facts that a later ignition point leads to a smaller torque and, conversely, that an earlier ignition point leads to a higher torque. The ignition point during the idling operation is set "late". The torque can, therefore, be rapidly increased during an idling speed reduction by adjusting the ignition point from "late" to "early" to thereby prevent the idling speed from falling short of the critical speed.
However, the internal combustion engine inherently runs with lower efficiency when a late ignition point is set than when an early ignition point is set. For this reason, idle speed control with ignition intervention also leads to relatively high fuel consumption.
EP 0 743 211 A2 concerns a stop procedure of the internal combustion engine in a hybrid vehicle.
DE 34 42 112 C2 concerns gear shunting during fuel cutoff in the overrun to drive secondary units with the axle drive of the vehicle.
DE 43 25 505 A1 and DE 41 08 751 C2 concern control types of the generator of a vehicle in which the generator is controlled to reduce braking torque in the event of load engagement in order to contribute to keeping the idle speed constant. Engagement of electrical loads is involved in the first-mentioned document.